What you need to do is first decide on the graphics card you want. Then research the power requirements for your whole computer with this new card and select a suitable supply from a reputable maker to support the new demands. I recommend you start here,
Toms Hardware - Best Graphics Cards for the Money, January 2012 and find the card you want. Then use the
eXtreme PSU Calculator Lite to determine your minimum and recommended power supply unit (PSU) requirements. Plan ahead and plug in all the hardware you think you might have in 2 or 3 years (extra drives, bigger or 2nd video card, more RAM, etc.). Be sure to read and heed the notes at the bottom of the calculator page. I recommend setting Capacitor Aging to 10% and setting both TDP and system load to 100%. These steps ensure the recommended supply has adequate head room for stress free (and perhaps quieter) operation, as well as future hardware demands. Setting Capacitor Aging to 30% will provide an even nicer amount of headroom. And remember, the computer’s components will only draw what they need, not what the PSU is capable of delivering. And the PSU will only draw from the wall what the computer demands, plus another 15 - 20% due to PSU inefficiencies. Buying way too big hurts
only the budget. Make sure you buy a supply from a reputable maker and that it is
80 PLUS certified.